Technical Field
The disclosure relates to a light emitting device with a light emitting element mounted thereon, and more particularly, to a light emitting device on which a light emitting element is mounted while its surface with electrodes is facing upward (hereinafter referred to as the face-up mounting type).
Description of Related Art
Semiconductor light emitting elements (hereinafter referred to as a “light emitting element”), such as a light emitting diode (LED) and a laser diode (LD), have the advantages of being small in size with a high power efficiency, emitting the vibrant colored light, having less possibility that a bulb will burn out because of the use of the semiconductor element, and having excellent initial driving characteristics and great resistance against repeated use, including vibration, and turning on/off.
As the light emitting element has such excellent advantages, the light emitting device with the light emitting element mounted as a light emitting source is used as alight source of alighting equipment or a backlight of liquid crystal display (LCD) for general consumer use. Thus, the light emitting device is designed so as to be suitable for these uses.
For example, a light emitting device may be used which includes a light emitting element mounted on a wiring board (printed substrate) with patterns of lead electrodes formed of a metal film on the surface of a plate-like or film-like substrate. Mounting a semiconductor element (chip) on the wiring board involves mounting the semiconductor element in a predetermined mounting region of the wiring board, electrically connecting electrodes of the semiconductor element to the lead electrode (inner lead) on the wiring board. After the mounting, the semiconductor element may be sealed the semiconductor element with resin. The manner for mounting a semiconductor element is classified into a face-up mounting which includes placing the semiconductor element so as to cause its surface with at least one electrode (pad electrode) to face upward and forming a wiring (conductive line) for electrically connecting the pad electrode to the lead electrode, and a flip-chip mounting (face-down mounting) which includes mounting the semiconductor element while the pad electrodes are facing down and coupled to the lead electrodes.
In the face-up mounting, a wiring is normally formed by wire bonding.
At this time, a wire serving as the wiring is provided in an arc to have only its both ends bonded to the pad electrode of the semiconductor element and the lead electrode. Thus, in the wire bonding mounting, the wire sealed by resin together with the semiconductor element might be broken by a stress from the outside of the light emitting device and so on. On the other hand, in the flip-chip mounting, the lead electrodes on the wiring board need to be positioned facing to each other depending on the position of the pad electrode of the semiconductor element. Thus, the flip-chip mounting is not easy as compared to the wiring bonding mounting.
Accordingly, methods for mounting a face-up mounting type light emitting element on a wiring board without using wire bonding have been developed. For example, JP 2011-243666 A discloses a mounting method for connecting electrodes on the upper surface of a light emitting element to lead electrodes of a wiring board by printing a wiring with conductive ink. The conductive ink has been recently applied for formation of fine wirings, such as a wiring board, and can be printed on a surface with certain unevenness, especially, by an ink jet method. Thus, the use of the conductive ink can form wirings (die wires) from the electrodes on the upper surface of a chip of the light emitting element through the sides (end surfaces) of the chip to the wiring board which is a chip mounting surface.
In the mounting method disclosed in JP 2011-243666 A, a layer made of translucent resin, such as epoxy resin or urethane resin, is provided on the side surfaces of the light emitting element mounted on the wiring board, whereby the die wire formed of the conductive ink is insulated from a semiconductor layer of the light emitting element. In this structure, however, the die wire is sandwiched between a resin layer provided on the side surface of the light emitting element and the translucent resin for sealing the light emitting element. That is, the die wire is supported only by the resin, and as a result, might be broken in the same way as in the wiring bonding mounting.
Particularly, in use of the film-like flexible substrate as the wiring board, the wiring can be easily broken due to a bending stress of the flexible substrate. The resin layer provided on the side surface of the light emitting element is sandwiched between the light emitting element and the die wire having a high thermal conductivity. The resin layer is likely to be degraded by heat or light as the use of the light emitting device, and thus might reduce its translucency.